Site-directed mutagenesis was used to investigate the loading of iron into rat liver ferritin by ceruloplasmin. Changes were made in the H chain to investigate the role of tyrosines involved in an inherent ferroxidase activity thought to be involved in the self-loading of iron into ferritin. Mutation Y34F affected the rate of iron loading by ceruloplasmin and incorporation of the oxidized iron into the core. Mutation Y29R (making it analogous to the L chain) had no effect on iron oxidation but slightly decreased core formation. A double mutation in the L chain, to open the α- helix bundle channel, and R25Y, making the protein more analogous to the H chain, increased the amount of iron incorporated into the core, again suggesting that this Tyr is involved in ligand exchange for core formation. Additional changes in the L chain involving the BC loop suggest that the entire BC loop is involved in the association of ferritin with ceruloplasmin, increasing its ferroxidase activity and the rate of iron loading into ferritin.
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